1. Field of the Invention
This invention relates to a light beam deflecting device utilizing a supersonic wave.
The supersonic wave deflector (hereinafter referred to as "A/O deflector") is capable of performing high speed scanning in comparison with heretofore known mechanical deflectors such as a rotational polygonal mirror, a galvano-mirror, and so forth. On account of this, the A/O deflector has been expected for its use in various fields such as a high speed laser beam printer, a TV display device, and so on.
2. Description of the Prior Art
The A/O deflector is classified into two types, i.e., a volume type deflector as shown in FIG. 1 of the accompanying drawing, and a thin film optical waveguide type deflector as shown in FIG. 2. The present invention is applicable to both types.
In the following, the construction and function of these A/O deflectors will be explained. The volume type deflector shown in FIG. 1 is constructed with a supersonic wave transducer 1 composed of a piezoelectric substance such as PLZT, etc. This transducer 1 is adhesively disposed on a supersonic waveguide member such TeO.sub.2, etc. When a high frequency voltage in a frequency band ranging from 50 MHz to 1,000 MHz is applied from outside to this transducer 1, the supersonic wave propagates through the waveguide member 2 in the form of compression waves, whereby a diffraction lattice structure is formed in the waveguide member due to variations in the refractive index. When a laser beam 3 is introduced into this waveguide member 2, it is subjected to the Bragg's diffraction by the abovementioned diffraction lattice structure and is projected out as a diffracted light beam 4. In this case, an angle of diffraction .theta. formed by a zero order diffracted light beam 5 and the diffracted light beam 4 as projected out fluctuates in accordance with the frequency of the high frequency wave to be applied to the transducer 1, which is given by the following equation: ##EQU1## (where: V is a speed of the supersonic wave; f is an applied frequency; .lambda. is a wavelength of the incident light beam in the air; and n is a refractive index of the waveguide member (or medium) 2).
From the above equation, it will be understood that, by varying the frequency f of the applying signal, the angle of diffraction .theta. can be varied and the projecting light beam 4 can be deflected and scanned. The maximum deflection in this deflecting angle is governed by selection of an angular range for the Bragg's diffraction. That is to say, when the frequency of the high frequency wave exceeds a certain definite frequency value, the incident light beam comes out of the coupling condition for the diffractive lattice structure resulting from a supersonic wave field, and the diffracting efficiency lowers. On account of this, the maximum deflecting angle (a scannable angular range) is limited, which is 3 degrees or so at most in the conventional device.
FIG. 2 illustrates the thin film waveguide type A/O deflector. An optical waveguide 7 is formed by diffusing titanium on the surface of a piezoelectric crystal substrate 6 such as LiNbO.sub.3, etc. The optical waveguide has a thickness of approximately 2 micro-meters or so, and constitutes a high refractive index layer with its refractive index being higher by approximately 0.01 than the refractive index of the LiNbO.sub.3 substrate (R.I. of 2.2).
By moving a prism 8 of a high refractive index near to this optical waveguide 7, a laser beam 9 is introduced into the waveguide 7 from outside. A comb-shaped electrode 10 for supersonic wave excitation is provided on the surface of the optical waveguide 7. By application of a high frequency wave to this electrode 10, a supersonic surface wave is generated on the surface of the substrate 6. A light beam 11 projected into the optical waveguide 7 is diffracted by this supersonic surface wave as a deflected light beam 12. As in the case with the volume-type A/O deflector, the light beam 12 is deflected by varying the frequency of the high frequency wave to be applied to the electrode 10. The deflected light beam 12 is projected outside by a projecting prism 13, and used as the deflected light beam. With this optical waveguide type A/O deflector, too, the maximum deflection is limited by the coupling conditions for the Bragg's diffraction.
In the field of the optical waveguide type A/O deflector, there have been several attempts made to increase the angle of deflection. One example is mentioned in detail in the following publication: IEEE Transactions on Circuit and Systems, Vol. CAS-26, No. 12, p 1072, "Guided-Wave Acousto-Optic Bragg Modulations for Wide-Band Integrated Optic Communications and Signal Processing" by C. S. Tsai. One of the actual examples is as shown in FIG. 3, wherein the deflector is so constructed that a wide frequency band is shared by a plurality of transducers 14.sub.1, 14.sub.2, . . . , each having a different resonant band, and that, by slightly tilting the transducers one after the other, the incident light beam may be coupled in the entire frequency band. In the illustrated example where four transducers are used, a frequency band of 680 MHz is obtained. In this case, the angle of deflection is approximately 4 degrees.
The other example is as shown in FIG. 4, wherein the device is so constructed that a pitch P and a inclination .phi. of each tooth of a comb-shaped electrode 15 are sequentially varied to change the travelling direction of the supersonic wave as it proceeds from a low frequency range to a high frequency range, thereby coupling the incident light beam in a wide frequency band.
Even in these improved deflectors, the maximum angle of deflection remains 4 degrees or so due to various restrictions imposed on its manufacture such as a driver technique, pattern working technique for the comb-shaped electrode, and so forth.
While this method of increasing the angle of deflection necessitates a voltage applying oscillator of a wide frequency band with increase in the angle of deflection, it is generally hard to obtain such an oscillator. As one way of widening the angle of deflection without broadening the frequency band, there may be contemplated a method, wherein, as shown in FIG. 5, a plurality of transducers, each having the same frequency band, are disposed with their angle of inclination being varied, and the directions of the incident light beams are also varied corresponding to the angle of inclination of each transducer so that the combinations between the incident light beams and the transducers may be sequentially changed over. An advantage to be derived from this method is that the frequency in the supersonic transducer does not become extremely high. A problem with this method, on the other hand, is the influence of the zero order diffracted light which exists in this kind of deflector.
In more detail, as in FIG. 5, when the first light beam 18 is projected to the supersonic wave 17 excited by the first transducer 16, a part of the light beam is deflected, while the other part becomes the zero order light 19 without being diffracted. By this first transducer 16, an angular range 21 (as diagonally shaded) between the maximum diffraction angle 20 and the minimum diffraction angle 24 is scanned. Then, by combination of the second transducer 22 changed over time-sequentially from the first transducer 16 and the incident light beam 23, a deflecting range 25 (as diagonally shaded) between the maximum diffraction angle 24 and the minimum diffraction angle 26, which is contiguous to the first delecting range 21, is scanned. In this deflecting range 25, the zero order light beam 19 derived from the abovementioned first light beam 18 is present. On account of this, when this deflector is used for the laser beam recording or the laser beam display, this zero order light beam 19 constitutes a stationary light beam with respect to the deflected light beam, so that, even when the light quantity is rather small, it remains a bright line noise in the deflecting ranges 21, 25, which is very harmful to the recording or the display.
In the following, explanations will be given as to the fly-back time of the deflected light beam. Conventionally, when the light beam scanning is to be conducted using the A/O deflector, an incident light beam is repetitively deflected and scanned from a position a to a position b as shown in FIG. 6A by introducing the so-called "chirped signal" which repeatedly changes from a low frequency f.sub.1 to a high frequency f.sub.2 as shown in FIG. 6B. In this case, the fly-back time required for the deflected beam to return from its maximum diffraction angle position b to its minimum diffraction angle position a is given by a time .tau., during which a supersonic wave of a speed V passes through a width W of the light beam (.tau.=W/V). As one example, the time .tau. for the supersonic wave of a speed of 3.5.times.10.sup.6 mm/sec. to cross the light beam of 6 mm in width is 1.7 micro-second, which fly-back time is problematically non-negligible in the high speed scanning. Accordingly, the fly-back time should desirably be zero.